The standard commercial lighting discharge device, such as, for example, fluorescent lamps and high pressure mercury vapor lamps, are characterized as negative resistance devices. During operation of these devices, a non-linear relationship exists between the current through the device and the voltage across the device. As used herein, the term "lamp" is intended to include gaseous discharge lamps such as high intensity discharge lamps and fluorescent lamps. Such lamps operate more efficiently in frequencies higher than 60 Hz. Typically, such frequencies may range from 15 KHz to as high as 100 KHz.
In order to obtain longer life and constant brightness for lamps designed for high frequency operation, the lamp current must be regulated to a higher degree than with prior ballast circuits. Problems exist in obtaining a highly regulated lamp current because, in a ballast intended for residential or commercial use, conventional 60 Hz line voltage is the typical power source. Even when full-wave rectified, so that a 60 Hz source in effect becomes a 120 Hz source, there is substantial variation in the amplitude of the source voltage fed to the power transformer which normally energizes the lamp load. If this variation in voltage amplitude is reflected in applied lamp current, an undesirable situation exists in reducing effective lamp life as well as resulting in uncontrolled variations in lamp brightness. Additionally, it is desired to operate multiple lamps utilizing a single power source. Due to the differences in operation of the individual lamps within a multi-lamp configuration, variations in lamp brightness will also result.
A need has thus arisen for a control device for gaseous discharge lamps operating in multiple lamp configurations such that lamp circuits operate over a wide variety of supply voltage levels without substantial fluctuations in the total power employed by the system to improve lamp life and provide constant brightness output of the individual lamps within the system.